A perovskite structure La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM), is a promising support material for dual − phase membrane for high − temperature CO2 separation owing to its high oxygen ion conductivity. The microstructure plays an important role in CO2 separation performance of the membrane. The result shows that effective particle − to − particle contact is a crucial guarantee for improving CO2 permeability. The uniformly distributed small pores of the support can greatly reduce the leakage. The Al2O3 coating on the porous support can improve the wettability between LSGM and molten carbonate, simultaneously increasing CO2 permeation flux while reducing leakage. The Al2O3 coated LSGM − based dual − phase membrane achieves a CO2 permeation flux of 0.34 mL min−1 cm−2 at 750 °C, with no detectable leakage. Above 750 °C, a reaction occurs between LSGM and molten carbonate, leading to a decline in membrane performance. For LSGM − carbonate dual − phase membranes, it is recommended to operate at temperatures not exceeding 750 °C.